The present invention relates to apparatus and method for directly measuring ash fusion properties at elevated temperatures and pressures, and more specifically to a high-pressure microdilatometer (HPMD) which measures ash fusion and sintering behavior by independent but simultaneous measurement of expansion/contraction characteristics and electrical resistivity of ash samples at elevated temperature and pressures in oxidizing or reducing atmospheres.
In coal combustion and gasification systems such as fluidized beds, slagging fixed-beds and entrained-flow systems the fusion, sintering, and deposition of ash impose serious operating problems which have been difficult to cope with or overcome. These problems are becoming even more difficult to understand so that suitable corrections may be made due to the trend in using coal conversion systems which operate at relatively high temperatures and pressures. Coal ash has different characteristics when subjected to high temperatures and pressures. For example, in a slagging fixed-bed gasifier coal becomes devolatilized in a highly reducing atmosphere, but the coal ash undergoes fusion at elevated temperature and pressure in an oxidizing atmosphere. The fusibility and sinterability of coal ash critically affect slagging and, hence, fouling in combustors and gasifiers.
Coal ash fusibility has been previously determined by an ASTM test used for evaluating the slagging tendency of coal ash by measuring gross changes in shape of a conical compact of coal particles heated at 80.degree. C. per minute (at 1 atm pressure) in a specified atmosphere. Four characteristic temperatures defining ash fusibility were based on the deformation of the cone with rising temperature: (1) initial deformation temperature where the apex of the cone first becomes rounded; (2) softening temperature where the cone fuses and the height is equal to the base; (3) hemispherical temperature where the height of the cone is equal to half of the base width; and (4) fluid temperature where ash flows into a fluid layer.
It was found that the ASTM technique yielded only the gross tendencies of bulk samples with data likely applying to large ash particles in the conical compact. The "low melting" components providing minor concentrations in the ash may lead to particle-to-particle bonding of fly ash below the melting point temperature of bulk ash so as to prevent the ASTM technique from revealing the fusion and melting behavior of minor, e.g. alkali, components in the ash. Also, the ash melting and fusion process may occur at temperatures differing form those observed by the ASTM technique.
It has been shown that ash resistivity drops suddenly when the temperature of the ash reaches a certain transition temperature, (T.sub.r) as discussed in Cumming et al, "An Electrical Resistance Method for Detecting the Onset of Fusion in Coal Ash, "Fouling and Slagging Resulting from Impurities in Combustion Gases, R. W. Bryers, ed., New York: Engineering Foundation, 1983, pp. 329-341. This transition temperature which indicates the first presence of a trace liquid phase is invariable below the temperature where initial deformation of the conical compact occurred when using the ASTM technique.
The comparison of sintering point data from both volume change and electrical resistivity measurements has also been used. The sintering points of several coal ashes, except for high sodium North Dakota lignite, have been found to agree closely. Electrical resistivity measurements for high-sodium coal indicates a much lower sintering temperature which was possibly due to an Na.sub.2 O-induced liquid phase. This technique provided a valuable tool for determining sintering effects due to addition or removal of mineral constituents such as Na.sub.2 O, and provided for the assessment of models describing vitrification. However, it has been found that no presently available techniques or equipment can provide accurate measurement of the behavior of coal ash at elevated temperatures and pressures